In recent years, in diagnoses in the medical field, specimen analysis tools for detecting pathogens such as bacteria and viruses or determining the presence or absence of pregnancies are in widespread use. Among these specimen analysis tools, a specimen analysis tool utilizing an immunochromatography method is used widely because it enables detections to be conducted easily and rapidly. The principle of detection by the immunochromatography method performed using the specimen analysis tool is as follows, for example. First, a specimen analysis tool formed of a porous base material in which immobilized antibodies are immobilized on a detection portion of the porous base material is provided. A sample (a specimen) and antibodies labeled with colored particles are added thereto. When antigens as a target component are present in the sample, the labeled antibodies and the immobilized antibodies form complexes via the antigens, whereby the detection portion on the porous base material is colored by the colored particles. The label may be, for example, in addition to the colored particles, a combination of an enzyme and a substrate that is colored by an enzyme reaction. The detection portion generally is in a line form. When coloring is observed, it is regarded that the target component is present in the sample, so that it is determined that the sample is positive. When no coloring is observed, it is regarded that no target component is present in the sample so that it is determined that the sample is negative. In the immunochromatography method, it has been attempted that plural detection portions are provided and a target component in a sample is semiquantitatively detected by observing stepwise coloring (see, for example, JP H8-278305 A).
In the detection of a target component using an immunochromatography method, a prozone phenomenon is generated in the case where the concentration of the target component is high, which is a problem. The prozone phenomenon is a phenomenon in which although the actual concentration of the target component in a sample is high, it is apparently determined that the target component is not present or the concentration thereof is low. FIG. 5 is an illustration showing a relationship between the concentration of antigen and the absorbance (the degree of coloring) in a detection portion in an immunochromatography method. The absorbance is increased with an increase in the concentration of the antigen. However, when the concentration reaches a certain concentration or higher, a phenomenon in which an absorbance cannot be determined (coloring does not occur) is generated. Therefore, for example, when the concentration of antigen is Z1, an absorbance X is detected, and when the concentration of antigen is Z2 in a high concentration region, the same absorbance is also detected because a prozone phenomenon is generated. This phenomenon is caused by the fact that when the excess amount of the target component is present in a sample, the immobilized antibodies are blocked by the target component that has not reacted with labeled antibodies, so that the target component that has reacted with the labeled antibodies cannot be captured by the immobilized antibodies. Therefore, in an analysis, there is a case that a sample with a concentration in the high concentration region is apparently determined as negative (i.e., a false-negative), and it is difficult to distinguish a false-negative from an actual negative. When a false-negative caused by a prozone phenomenon is suspected, it has been required that a sample is diluted as appropriate and then re-examination is performed. On the other hand, it has been proposed to detect a prozone phenomenon that plural types of antibodies, which are different in affinity from one another are used as immobilized antibodies in a specimen analysis tool (see, for example, WO 2003/014740) and stepwise increased amounts of antibodies are applied to the plural detection portions (see, for example, JP No. 3644780).